INCOMMENSURATE-COMMENSURATE TRANSITION IN K2ZNBR4 AND K2COBR4

The structural phase transitions of K2ZnBr4 and K2CoBr4 are investigated by means of X-ray scattering. The transition from the room-temperature alpha-phase to the high-temperature beta-phase takes place at about 200-degrees-C. It is found that the beta-phase is an incommensurate one with the modulation wave vector q = (1/3 - delta)c*, which is followed by a normal phase of Pmcn above 288-degrees-C (Zn-salt) or 282-degrees-C (Co-salt). If the beta-phase is supercooled to below room temperature, then the crystal is a commensurate phase with q = 1/3c*. With increasing temperature, the commensurate-incommensurate phase transition is observed at 5-degrees-C in Zn-salt. The transition sequence of normal, incommensurate and commensurate phases is quite common in A2XY4-type incommensurate materials, though the alpha-phase is very different from others.